Apparatus and method of fluid level measurement

ABSTRACT

An apparatus for fluid level measurement comprises an upright tube ( 10 ) extending into a storage tank ( 12 ) substantially the full height distance of the tank ( 12 ). An acoustic transducer ( 14 ) is positioned at the top of the tube ( 10 ) and adapted to emit a sound pulse to be bounced off the surface ( 16 ) of the fluid level within the tube ( 10 ) and to receive a reflected pulse. Means is provided for measurement of the time of arrival of the reflected sound pulse. An acoustic coupling means is provided for the transducer ( 14 ) to shape and increase the amplitude of the pulse emitted from the transducer ( 14 ) together with an increase in the amplitude of the voltage produced by the transducer ( 14 ) in response to the received reflected pulse.

[0001] This apparatus relates to an improved apparatus for fluid levelmeasurement in a storage tank. Fluid level measurement is an importantrequirement in monitoring volume of liquid in a tank in the assessmentof when it will require replenishment. The invention relates to themeasurement of distance from a datum point to a fluid level in the tankby means of reflection of sound waves.

[0002] A disadvantage of existing apparatus is its lack of accuracy. Anobject of the present invention is to obviate or mitigate thisdisadvantage.

[0003] Accordingly, the present invention is an apparatus for fluidlevel measurement comprising an upright tube extending into a storagetank substantially the full height distance of the tank, an acoustictransducer positioned at the top of the tube and adapted to emit a soundpulse to be bounced off the surface of the fluid level within the tubeand to received a reflected pulse, means for measuring of the time ofarrival of the reflected sound pulse, and an acoustic coupling means forthe transducer to shape and increase the amplitude of the pulse emittedfrom the transducer together with an increase in the amplitude of thevoltage produced by the transducer in response to the received reflectedpulse.

[0004] Preferably, the acoustic transducer is an assembly of a disc ofpiezo-electric material bonded to a larger disc of thin metal, theassembly generating sound in response to an applied voltage, andgenerating a voltage when the disc is vibrated by sound impinging on it.

[0005] Preferably also, the acoustic coupling means comprises a stopperfor the upper end of the tube, the stopped having a conical passageaxially thereof, the passage having a restricted throat at its upper endan opening having the same diameter as the tube at its lower end. Thetop of the stopper beneficially has a cylindrical recess of shallowdepth, the throat of the passage axially entering the bottom of therecess and the transducer sealing off the top of the recess whereby therecess is air-tight except for the throat.

[0006] An embodiment of the present invention will now be described, byway of example, with reference tot he accompanying drawings, in which:

[0007]FIG. 1 is a diagrammatic side view of part of an apparatus forfluid level measurement according to the present invention;

[0008]FIG. 2 is a cross-sectional view of a detail of the apparatus atan upper end and to a larger scale;

[0009]FIG. 3 is a driving and receiving electric circuit for theapparatus; and

[0010]FIGS. 4A and 4B are two graphs of voltage against timerespectively showing the state of detection of an early soundreflection, and of a later sound reflection.

[0011] Referring to the drawings, an apparatus for fluid levelmeasurement comprises an upright tube 10 extending into a storage tank12 substantially the full height distance of the tank 12. An acoustictransducer 14 positioned at the top of the tube 10 and adapted to emit asound pulse to be bounced off the surface 16 of the fluid level withinthe tube 10 and to receive a reflected pulse. Means is provided formeasuring of the time of arrival of the received reflected sound pulsecompared to the time of emission of a sound pulse. An acoustic couplingmeans is provided for the transducer 14 to shape and increase theamplitude of the pulse emitted from the transducer 14 together with anincrease in the amplitude of the voltage produced by the transducer 14in response to the received reflected pulse.

[0012] An acoustic transducer 14 is an assembly of a disc 14A ofpiezo-electric material bonded to a larger disc 14B of thin metal, theassembly generating sound in response to an applied voltage, aridgenerating a voltage when the disc 14B is vibrated by sound impinging onit. When a voltage is applied across the thickness of the disc 14A ofpiezo-electric material, it causes the disc 14B to bend, thus creating adeflection of the disc 14B from its resting position. A suddendeflection of the disc 14B causes a corresponding sound pulse to betransferred to the air surrounding the disc 14B. When the disc 14B issubject to a varying pressure due to sound waves, this causes the disc14B to bend from its resting position, and the piezo-electric materialthen produces a voltage. A characteristic of this type of transducer ismechanical resonance. When a short pulse of voltage or sound is applied,the disc 14B vibrates at one or more characteristic frequencies. Whenthe disc 14B is not subject to external mechanical resistance ordamping, the vibrations typically last for many cycles. This is anundesirable characteristic in the context of this invention. The resultof these extended vibrations is to make it difficult to determine theexact position in time of the acoustic pulse, so causing measurementuncertainties. This difficulty is overcome by the acoustic couplingmeans described herein.

[0013] The acoustic transducer is coupled to the, tube 10, such thevibration of the transducer causes sound waves to be launched in the airin the tube 10. Reflected waves coming back up the tube 10 vibrate thetransducer 14. The transducer 14 is mounted above the highest expectedliquid level in the tube 10. The diameter of the tube 10 is smallcompared to the range of wavelengths contained in the sound pulse, whichmeans that the pulse travels in the tube 10 at the characteristic speedof sound. A second pulse generated at one end of the tube 10 travelswith little attenuation down the tube 10, until it encounters adiscontinuity. At the discontinuity, a proportion of the incident waveis reflected back up the tube 10, again experiencing little attenuationon the way back. When the discontinuity consists of a liquid surface 16,then because the liquid is far more dense than air, almost all theincident energy is reflected back up the tube 10. A breather hole 18 isprovided in the tube 10, above maximum level of fluid allowed in thetank 12. This hole 18 allows the air pressure in the tube 10 to equalisewith that of the surrounding air as the liquid level rises and falls inthe tube 10. Some attenuation of sound occurs as it travels down thetube 10, this being greater as the tube 10 diameter is reduced. Theattenuation is due to frictional losses near the wall of the tube 10 andis a fairly well defined exponential function of the distance travelledby the pulse. This effect can be compensated for by electronic means.

[0014] The acoustic coupling means comprises a stopper 20 for the upperend of the tube 10. The stopper 20 has a conical passage 22 axiallythereof with the passage 22 having a restricted throat 24 at its upperend an opening 26 having the same diameter as the tube 10 at its lowerend. The top of the stopper 20 has a cylindrical recess 28 of shallowdepth with the throat 24 of the passage 22 axially entering the bottomof the recess 28 and the outer part of the disc 14B of the transducer 14sealing off the top of the recess 28 whereby the recess 28 is air-tightexcept for the throat 24. The stopper 2Q is sealed to the inside of thetube 10. The recess 28 when closed off by the disc 14B is shallow sothat the air inside the cavity formed compresses only slightly when thedisc 14B bends, thus efficiently transferring air pressure from the disc14B to the throat 24. The effect of the cavity and the stopper 20 is toamplify the movement of air at the start of the tube 10, compared tocoupling the transducer 14 to the tube 10 without the acoustic couplingmeans. The amplification effect of the acoustic coupling means isbeneficial to the invention as the increased sound pressure in the tube10 helps to swamp out the undesirable external sounds that mightotherwise cause measurement errors. The amplification works equally wellfor the received reflected pulse, so increasing the resultant receivedvoltage.

[0015] The acoustic coupling means provides mechanical damping to thesurface of the transducer disc 14B thus damping out the undesirableresonances of the disc 14B. This mechanical damping arises because theimproved coupling causes significant energy to be radiated down the tube10 when the disc 14B moves, thus causing the disc 14B to experience asignificant mechanical resistance.

[0016] In practice, the acoustic coupling means produces resonances ofits own, in addition to the resonances of the transducer disc 14B. Theeffects of these can be minimised by the appropriate choice of length ofstopper 20, diameter of throat 24 and cavity volume in the closed recess28.

[0017] When the resonances are well damped by the acoustic couplingmeans, the shape of the received pulse tends towards a shorter durationimpulse, with a well-defined position in time. In practice, some loweramplitude oscillations can be tolerated before the main impulse,provided subsequent electronic means can discriminate between theoccurrence of the main pulse, and the earlier “spurious” pulse.

[0018] To generate an impulsive force to the piezo-electric disc 14A, anarrow pulse of voltage is applied to the piezo-electric disc 14A. Sucha waveform may readily be generated by a logic circuit ormicrocontroller logic output. The pulse width has some effect on boththe amplitude and shape of the subsequent received pulse. This can beadjusted empirically.

[0019] Since the transducer 14 is used for receiving as well astransmitting pulses, the pulse generator needs to disconnect from oneterminal of the piezo-electric disc 14A after the pulse has beenlaunched. The other terminal of the piezo-electric disc 14A is held at aconstant voltage when ready to receive pulses. When using a logiccircuit or a microcontroller 30 (as shown), this disconnection ofone-terminal can be achieved by setting a relevant output pin (notshown), to an open-circuit state.

[0020] The pulse detector consists of a voltage comparator 32 with oneinput connected to the “free” terminal of the piezo-electric disc 14A.The other input of the voltage comparator 32 is connected to a referencevoltage 34. When the amplitude of the received pulse exceeds thereference voltage, the output of the comparator 32 changes state. Thischange of state is used to trigger subsequent logic or timing means ofthe microcontroller 30.

[0021] The means for measuring the time of arrival of the receivedreflected sound pulse compared to the time of emission of a sound pulseis a facility incorporated into the microcontroller 30.

[0022] To measure the distance from the transducer 14 down the tube 10to the liquid surface 16, a timing circuit is used to measure the timeinterval from the end of the driving pulse, to the arrival of the firstreflected pulse, as indicated by the pulse detector. This distance inmetres is given by ct/2, where c is the speed of sound in air (343metres per second at 20° centigrade), and t is the measured timeinterval in seconds.

[0023] The distance to liquid level readings is then converted intoliquid volume readings. In a typical application of the invention, thevalue of the measured distance to the surface of the fluid is convertedinto a fluid volume reading. The conversion formula depends upon thesize and shape of the tank containing the fluid.

[0024] Compensation for the attenuation of the sound as it travels downthe tube 10 can be provided for as well as compensation for variationsin readings due to ambient temperature changes. As the sound pulsetravels down the tube 18, it is attenuated due to frictional effects onthe walls of the tube. The reflected sound pulse is also attenuated. Theamount of attenuation increases with the length of the tube. Theattenuation is an exponential function of the distance travelled by thesound pulse.

[0025] The attenuation of the sound pulse in a long tube means that thecomparator 32 will fail to detect pulses that are attenuated so thattheir level is below the threshold on input 34 of the comparator. Toeliminate this undesirable effect, means is provided to reduce thethreshold on input 34 with time, such the pulses received in a shorttime are detected with a high threshold, and pulses that are received ina long time are detected with a lower threshold.

[0026] The amplitude of a sound pulse travelling in a tube is a negativeexponential function of the distance travelled. This means that theamplitude of the received pulse is a negative exponential function ofthe time of arrival. To compensate for the attenuation of the soundpulse, the threshold voltage should therefore be decreased exponentiallywith time, as shown in FIG. 4.

[0027] Exponential adjustment of the threshold with time can be providedby supplying the threshold input 34 from a capacitor. When the soundpulse is transmitted, the capacitor-is charged-up to a voltage thatprovides a threshold suitable for early reflected pulses. The capacitoris arranged to be discharged by a resistor, such that the thresholddecreases exponentially with time. The values of the capacitor andresistor are arranged such that the rate of decay of the thresholdvoltage matches the rate of decay of the reflected pulse amplitude.

[0028] The speed of sound in gases increases with increasing temperatureof the gas. This means that the measurement of distance by means ofmeasuring the time of arrival of a reflected sound pulse is subject toinaccuracy if the temperature of the gas is not constant.

[0029] To reduce this undesirable inaccuracy, the microcontroller 30 canbe provided with a temperature sensing means. The microcontollerdetermines the temperature of the gas in the tube 10, and calculates thecorrect distance value, based on both the time of arrival of thereflected sound pulse, and the measured temperature of the gas in thetube.

[0030] Alternatively, the inaccuracies in the measurement of distancedue to variations in the speed of sound may be reduced by providing asecond tube, stopped at the far end. The second tube is in the same gasatmosphere as the main tube, such that the speed of sound in both tubesis substantially the same. The second tube is equipped with the samesound production and detection means as the main tube. Themicrocontroller measures the time delay in the second tube. Since thedistance to the stopper end of the second tube is constant, the actualspeed of sound may be calculated from knowledge of the measured timedelay and the known length of the second tube. The mircrocontroller 30determines the speed of sound from a measurement of the time delay inthe second tube. It uses the measured speed of sound to determine thedistance to the fluid in the main tube. This method of compensation hasthe advantage that it does not require prior knowledge of thecharacteristics of the gas in which the measurement tube is immersed.This will be useful when the gas above the fluid is not air. If the gasis not air, the speed of sound is different from that of air. Forexample, the speed of sound in carbon dioxide is substantially lowerthan that in air.

[0031] Variations and modifications can be made without departing fromthe scope of the invention described above and as claimed hereinafter.

1. An apparatus for fluid level measurement comprising an upright tubeextending into a storage tank substantially the full height distance ofthe tank, an acoustic transducer positioned at the top of the tube andadapted to emit a sound pulse to be bounced off the surface of the fluidlevel within the tube and to receive a reflected pulse, means formeasuring of the time of arrival of the reflected sound pulse, and anacoustic coupling means for the transducer to shape and increase theamplitude of the pulse emitted from the transducer together with anincrease in the amplitude of the voltage produced by the transducer inresponse to the received reflected pulse.
 2. Apparatus as claimed inclaim 1, wherein the acoustic transducer is an assembly of a disc ofpiezo-electric material bonded to a larger disc of thin metal, theassembly generating sound in response to an applied voltage, andgenerating a voltage when the disc is vibrated by sound impinging on it.3. An apparatus as claimed in claim 1 or 2, wherein the acousticcoupling means comprises a stopper for the upper end of the tube, thestopper having a conical passage axially thereof, the passage having arestricted throat at its upper end and an opening having substantiallythe same diameter as the tube at its lower end.
 4. An apparatus asclaimed in claim 3, wherein the top of the stopper has a cylindricalrecess of shallow depth, the throat of the passage axially entering thebottom of the recess and the transducer sealing off the top of therecess whereby the recess is air-tight except for the throat. 5.Apparatus as claimed in claim 4, wherein the stopper is sealed to theinside of the tube.
 6. Apparatus as claimed in claim 4 or 5, wherein therecess when closed off by the disc is shallow so that the air inside thecavity formed compresses only slightly when the disc bends, thusefficiently transferring air pressure from the disc to the throat. 7.Apparatus substantially as hereinbefore described and as shown in theaccompanying drawings.